Fruit Valley Veterinary Clinic

Heartworms in Cats

The heartworm parasite, Dirofilaria immitis, is poorly adapted to infecting cats. Whereas dogs can harbor dozens, even hundreds, of fertile adult heartworms, the heartworm infection in cats tends to be abortive with only a few sterile worms. Dogs can support large numbers of worms while showing little or no outward signs of illness. In cats, the worms elicit a vicious inflammatory response that cause a syndrome of respiratory illness that resembles asthma. The worms usually die and cause possibly fatal embolisms. Although heartworm has been reported in indoor cats, this may be due to the often loose definition of an "indoor cat". Mosquitos are the required infective vector. Since treatment for subclinical heartworm disease in cats is NOT recommended, cats are not screened for heartworm by blood testing (also the incidence is likely quite low in Oswego). However, it is a very reasonable precaution to administer a heartworm preventive to cats from May through November. A good choice would be selamectin (Revolution topical) which also kill fleas, ear mites and intestinal parasites. We speculate also that selamectin may help protect against Cuterebra insect grub injury in cats.